1. Field of the Invention
This invention relates generally to filtering and, more particularly, to filters used in wireless communication systems.
2. Description of the Related Art
Base stations and user equipment in wireless communication systems typically communicate over the air interface by exchanging radiofrequency signals. Conventional base stations are often required to transmit signals with sufficient power to be detected and decoded by user equipment at distances that can exceed several kilometers. The base stations therefore implement power amplifiers to amplify signals for transmission using antennas coupled to the base station. Designers have been attempting to reduce power consumption in radio communication hardware and much attention has been focused on amplifiers.
Amplifiers are typically relatively inefficient. For example, conventional Class-A amplifiers can amplify typical Third Generation (3G) radiofrequency signals at efficiencies of less than 25% so that more than 75% of the power consumed by the Class-A amplifiers is wasted, e.g., by being dissipated as heat. The efficiency of amplifiers used in radio frequency communication can be improved by applying high efficiency linearity schemesincluding pre-distortion, Doherty designs, Envelope Tracking-Drain Modulation, linear amplification using nonlinear components (LINC), and the like. For example, Doherty designs use a second output stage as a peak amplifier to lift efficiency from the typical 15% up to 40-50% in a narrow to moderate bandwidth. For another example, Envelope Tracking designs can achieve efficiencies of up to 60% for narrow band signals by modulating the supply voltage to the amplifier in line with the envelope of the signal.
Class-S amplifiers can also operate in principle at very high efficiencies. In a Class-S amplifier, a digital representation of the transmitted signal is applied to inputs of a high power switching amplifier. The switching amplifier alternates between two states in response to the input digital signal. For example, the switching amplifier can alternate between a high current/no voltage state and a high-voltage/no current state. Class-S systems can be implemented as voltage switching or current switching systems. Theoretically, the Class-S amplifier can achieve 100% efficiency because the power dissipated in the amplifier is proportional to the product of the current and the voltage. Since either the current or the voltage is zero in both states, no power is dissipated and the amplifier operates at 100% efficiency. However, actual implementations of Class-S amplifiers operate at less than ideal efficiencies, at least in part because of difficulties associated with manipulating Giga-bit per second digital signals and resistive losses in the switching device.
Converting an analog signal to a digital signal introduces quantization noise artifacts and/or clock spurs at frequencies above and below the bandwidth of the input analog signal. Class-S amplifiers also amplify the high and low frequency quantization noise and/or clock spurs because they amplify the digital signal. Reconstruction filters are therefore added to the Class-S amplifier to filter out the noise before the amplified signal is applied to the antenna port. Conventional filters create a high reflection coefficient impedance to reflect the high and low frequency noise and thereby prevent noise from reaching the antenna port. Using an arbitrary topology reflection filter may decrease the efficiency of the amplifier by increasing the energy dissipated within the amplifier especially since these signals have a great deal of their total power spread over the entire frequency spectrum. For example, high-speed switching amplifiers that can operate at the gigahertz frequencies required for radiofrequency communication can be implemented using gallium nitride (GaN) field effect transistors. Energy can be dissipated in the channel region of these transistors and this channel loss can be exacerbated when noise is reflected from the reconstruction filter back into the switch.